1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor and a method for manufacturing a display device.
2. Description of the Related Art
In recent years, thin film transistors which are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface, such as a glass substrate, have been attracting attention. Thin film transistors have been widely used for integrated circuits (ICs) and electronic devices typified by electrooptic devices. Thin film transistors have rapidly developed particularly as switching elements for image display devices typified by liquid crystal display devices, electroluminescent (EL) display devices, and the like.
With the use of a polycrystalline semiconductor film, a thin film transistor with high mobility can be obtained. As examples of methods for forming a polycrystalline semiconductor film, a method in which an amorphous semiconductor film is crystallized with laser irradiation at a relatively low temperature, a method in which an amorphous semiconductor film is heated and crystallized at a temperature higher than that of the method in which an amorphous semiconductor film is crystallized with laser irradiation, and the like can be given. Such a crystalline semiconductor which is crystallized at a relatively low temperature is referred to as low temperature polysilicon (LTPS), and a crystalline semiconductor which is crystallized at a relatively high temperature is referred to as high temperature polysilicon (HTPS). In particular, since an LTPS is a polycrystalline semiconductor film which can be crystallized at a relatively low temperature, a glass substrate can be used, and various techniques have developed.
High productivity and reduction in the manufacturing cost have been required for the above method for manufacturing a thin film transistor. Simplification of a process is one way for increasing productivity and reducing the manufacturing cost. It is important that the number of photomasks to be used for a photolithography method be reduced in order to simplify a process. For example, when one more photomask is used, the following steps are further required: application of a resist, prebaking, light exposure, development, postbaking, and the like, and steps before and after the above steps, such as formation of a film, etching, removal of a resist, cleaning, and drying. Thus, the number of steps is significantly increased only by adding one photomask used in a manufacturing process of a thin film transistor. In order to reduce the number of photomasks used in a manufacturing process of a thin film transistor, many techniques have developed (for example, Patent Document 1 and Patent Document 2).